The present invention relates to a method for the formation of a silica-based coating film on a substrate or, more particularly, to an efficient method for the formation of a silica-based coating film having a relatively large thickness, which serves as a planarizing film, interlayer insulating film, protective film, orientation film, intermediate film in the multilayered resist method and the like indispensable in the manufacture of semiconductor devices, liquid crystal display panels and the like, on the surface of a substrate, in which the silica-based coating film formed by repeating coating works exhibits good adhesion between the coating layers and is free from defects such as pinholes.
It is an established prior art that a silica-based coating film is formed on the surface of various kinds of substrate materials with an object to serve as a planarizing film, interlayer insulating film, protective film, orientation film, intermediate film in the multilayered resist method and the like indispensable in the manufacture of semiconductor devices, liquid crystal display panels and the like.
Known methods for the formation of such a silica-based coating film include the vapor phase-deposition method and the coating method by using a liquid coating composition. The former method of vapor phase deposition is a method in which silica is deposited from vapor phase onto the substrate surface to cause growing of the deposited silica layer by using a special apparatus which is necessarily very expensive. Even by setting aside the problem of expensiveness of the apparatus used therefor, the vapor phase deposition method is not very efficient and productive when applied to mass production of semiconductor devices and liquid crystal display panels.
The latter method of coating is a method in which the substrate surface is coated with a solution of a partial hydrolysis-condensation product of an alkoxy silane compound followed by drying and baking of the coating layer to cause in situ formation of silica in the form of a coating film. This method is widely practiced in the electronic industry because the efficiency of the method is very high even without using any expensive apparatuses.
The coating solution used in the coating method for the formation of a silica-based coating film is prepared, according to the disclosure in Japanese Patent Publication 63-241076, for example, by dissolving a tetra alkoxy silane or an alkyl trialkoxy silane having a lower alkyl group in an organic solvent such as an alcohol solvent and hydrolyzing the alkoxy groups of the silane compound in the solution to form a partial hydrolysis-condensation product thereof in the presence of a catalyst. The coating solution of this type is useful in the fine patterning works for the manufacture of semiconductor devices of 4 to 16 mega DRAM where the fineness of the pattern is in the submicron order, i.e. about 0.8 .mu.m, or half-micron order, i.e. about 0.5 .mu.m, to give practically satisfactory results.
In the current status of the electronic industry, on the other hand, mass production is now required of semiconductor devices having a still higher degree of integration than 64 mega DRAM and 1 giga DRAM so that the fineness of the photo-lithographic patterning in the manufacture of semiconductor devices to exceed pattern resolution of 0.35 .mu.m or still finer.
From the standpoint of complying with the trend toward finer and finer pattern resolution, the silica-based coating film formed from the above mentioned coating solutions containing a partial hydrolysis-condensation product of an alkoxy silane compound has several problems in respect of adhesion to the upper layer thereon along with corrosion of the metallic circuit wiring layer resulting in short-circuiting between circuits to cause a decrease in the reliability of the semiconductor device. In addition, such a coating layer cannot completely fill up the interspace in the ultrafine patterns of 0.35 .mu.m or finer leaving so-called "blowholes".
The inventors conducted extensive investigations to solve the above mentioned problems and previously proposed a method for the preparation of a coating solution for the formation of a silica-based coating film in the ultrafine photolithographic patterning work in which the hydrolysis reaction of an alkoxy silane is conducted extensively in a silane solution of a low concentration. This coating solution, however, is defective because the thickness of the silica-based coating film obtained by a single coating work with the coating solution can hardly exceed 100 nm so that, when a silica-based coating film of larger thickness is desired, the coating work with the coating solution followed by drying must be repeated several times. Repetition of the coating works has another problem that, if not to mention the low productivity of the coating works, the adhesion between the coating layers is not always good enough along with eventual occurrence of pinholes as a result of repellency of the surface of the underlying coating layer.